A typical wireless communication system includes a number of base stations each radiating to provide one or more coverage areas or “sectors” in which to serve user equipment devices (UEs) such as cell phones, tablet computers, tracking devices, embedded wireless modules, and other wirelessly equipped devices, whether or not actually “user” operated. In turn, each base station may sit as a node on a core access network that includes entities such as a network controller, switches and/or gateways, and the core network may provide connectivity with one or more external transport networks such as the public switched telephone network (PSTN) and the Internet. With this arrangement, a UE within coverage of the system may engage in air interface communication with a base station and may thereby communicate via the base station with various remote network entities or with other UEs served by the base station.
Such a system may operate in accordance with a particular air interface protocol, examples of which include, without limitation, Long Term Evolution (LTE) (using orthogonal frequency division multiple access (OFDMA) and single-carrier frequency division multiple access (SC-FDMA)), Code Division Multiple Access (CDMA) (e.g., 1×RTT and 1×EV-DO), Global System for Mobile Communications (GSM), IEEE 802.11 (WIFI), and BLUETOOTH. Each protocol may define its own procedures for registration of UEs, initiation of communications, handover between sectors, and other functions.
Further, each base station may provide wireless service to UEs on one or more carrier frequencies, with the air interface defining various downlink and uplink channels. For instance, on the downlink, the air interface may define a reference channel for carrying a reference signal (also referred to as a pilot signal, or by other names) that UEs can measure to evaluate base station coverage quality, the air interface may define various other downlink control channels for carrying control signaling to UEs, and the air interface may define one or more downlink traffic channels for carrying bearer data and other information to UEs. And on the uplink, the air interface may define an access channel for carrying UE access requests to the base station, the air interface may define one or more other uplink control channels for carrying control signaling to the base station, and the air interface may define one or more uplink traffic channels for carrying bearer data and other information to the base station.
When a UE initially enters into coverage of such a system, the UE may scan for a best sector in which to operate, and the UE may then engage in signaling with the base station that provides that sector, to register for service. In particular, the UE may evaluate a reference signal receive quality (RSRQ) of each base station reference signal that the UE detects, where the RSRQ defines a signal-to-noise ratio of the reference signal (e.g., a ratio of reference signal receive power to noise power, also referred to as SINR, SNR, C/I, Ec/Io, or by other names), and the UE may select the sector having the highest RSRQ. Alternatively or additionally, the UE may evaluate another coverage quality metric, such as reference signal receive power (RSRP), of each base station reference signal, and the UE may select the sector having the best (e.g., highest) such metric, perhaps in combination with the highest RSRQ. The UE may then engage in signaling with the associated base station, to register for service.
The UE may then be served by the base station in a connected state or an idle state. In the connected state, the UE would have an established radio-link-layer connection with the base station through which the UE and base station could exchange bearer data (e.g., application-layer communications), facilitating communication by the UE on an external transport network for instance. And in the idle state, the UE would not have an established radio-link-layer connection with the base station but may periodically check for paging from the base station and may transition to the connected mode when necessary to engage in bearer communication.